/**
 * Retrieves value of a constant.
 * Based on code by Hugo Giraudel, http://www.sitepoint.com/dealing-constants-sass/
 * @param  {String} $name Name of constant.
 * @return {Misc}         Value of constant.
 */
@function const($name) {
  @if (map-has-key($CONSTANTS, $name)) {
    @return map-get($CONSTANTS, $name);
  }
  @error 'Unknown constant `#{$name}`.';
}